1. Field of the Invention
This invention relates to an exhaust gas purifying system for use in an internal combustion engine.
2. Description of the Prior Art
Exhaust gas recirculation systems have already been proposed which are designed to re-introduce a small amount of exhaust gases into the combustion cycle in order to reduce the generation of nitrous oxide. However, any attempt to increase the amount of exhaust gas re-introduced so as to reduce nitrous oxides or to rarefy the air-fuel mixture supplied to the engine so as to reduce nitrous oxides, hydrocarbons and carbon monoxide will result in poor drivability. Additionally, any attempt to enrich the air-fuel mixture supplied to the engine so as to maintain adequate drivability will result in increased hydrocarbons and carbon monoxide.
FIG. 1 is a graph showing the exhaust characteristics of prior art internal combustion engines. It is realized in FIG. 1 that nitrous oxides increase during acceleration. Thus, it is common practice to determine the amount of exhaust gases re-introduced on the basis of the condition appearing during acceleration. However, this causes an excessive amount of exhaust gases to be re-introduced during normal operation, which results in a drivability penalty.